Generally, the present invention relates to fiber optic communication systems, and particularly to fiber amplifiers for amplifying optical communications signals.
Optical fiber communications provides the capacity for transmitting large amounts of information. One way of realizing this large information handling capacity is to use wavelength division multiplexing (WDM). A WDM system uses a number of optical information channels, each channel operating at a different wavelength. Typically, each channel is generated by a separate laser source, the output from which is then modulated to carry the information. The modulated signals from different lasers is combined into one signal and is injected into a fiber link. After passing through the fiber communications system, the different channels are separated and then detected individually. Depending on the number of channels present, the bandwidth of the WDM signal can be very broad, several nm, if not tens of nm.
Optical communications systems are increasingly using fiber amplifiers for amplifying the optical communications signal, especially where the communications system transmits the optical signal over long distances. A commonly used type of fiber amplifier is the erbium-doped fiber amplifier (EDFA), where the core is doped with erbium ions. The erbium ions are excited by light from a pump laser, typically around 980 nm, passing along the fiber core. The excited erbium ions demonstrate a broad gain curve at around 1550 nm, the typical wavelength for an optical communications signal.
The erbium gain curve, however, is not flat, and so the channels of a WDM signal, spread out over a range of wavelengths, are typically amplified by different amounts in the EDFA. This is an undesirable effect: it is preferable to maintain all channels at approximately the same power level in order to maintain reliable and predictable performance. Furthermore, it is more efficient to spread the available optical power evenly among all channels, so that all channels can achieve the same minimum bit error rate (BER) without some channels having excess power.
One method of addressing the non-uniform gain of the EDFA is to use a gain flattening filter, whose loss spectrum complements the gain spectrum of the EDFA. The gain flattening filter has little loss for those channels that are amplified relatively weakly in the EDFA, but has higher loss for those channels that are amplified more. Therefore, when combined with the gain flattening filter, the EDFA may produce a spectrally flat output.
The gain flattening filter has a number of disadvantages. It introduces loss, thus reducing the overall efficiency of the optical communications system. Furthermore, the loss for each WDM channel is fixed. Therefore, the loss profile of the gain flattening filter cannot be adjusted dynamically to account for changing operating conditions.
There is a need, therefore, for a new approach to flattening the spectrum of the output from an EDFA, that avoids insertion loss and provides for dynamic adjustment.
The present invention is directed to a method and apparatus for controlling the gain tilt of a fiber amplifier, such as a pre/power fiber amplifier unit. The approach includes adjusting the gain tilt of one of the fiber amplifiers to at least partially compensate for the gain tilt in the other amplifier.
In one embodiment of the invention, a fiber amplifier system having a spectral gain characteristic, includes a first fiber amplifier having an input and an output, the first power amplifier exhibiting a first gain tilt, and a second fiber amplifier unit includes a second fiber amplifier. An output of the second fiber amplifier is coupled to the input of the first fiber amplifier. The second fiber amplifier exhibits a second gain tilt selected to compensate the first gain tilt so as to flatten the spectral gain characteristic of the amplifier system.
In another embodiment of the invention, a fiber amplifier system includes pre-amplifying means for pre-amplifying an optical signal and power amplifying means for power amplifying the pre-amplified optical signal received from the pre-amplifying means. The power amplifier system also includes means for adjusting spectral non-uniformity in the gain of the pre-amplifying means so as to flatten a spectral gain characteristic of the fiber power amplifier system.
In another embodiment of the invention, a method of operating a fiber power amplifier system, having a fiber pre-amplifier coupled to a fiber power amplifier, includes adjusting spectral non-uniformity in the gain of the fiber pre-amplifier so as to flatten a spectral gain characteristic of the fiber power amplifier system.
Another embodiment of the invention is directed to an optical information transmission system that includes an optical transmitter, an optical receiver; and an optical power amplifier unit disposed on an optical path between the optical transmitter and the optical receiver. The optical power amplifier unit includes a fiber power amplifier having an output coupled to the optical receiver and an input, the fiber power amplifier exhibiting a first gain tilt, and a fiber pre-amplifier having an input coupled to receive an input signal from the optical transmitter, and an output coupled to the input of the power amplifier. The fiber pre-amplifier exhibits a second gain tilt selected to compensate the first gain tilt so as to flatten the spectral gain characteristic of the amplifier system.
Another embodiment of the invention is directed to a fiber amplifier system that includes a pump laser emitting pump light; and a fiber amplifier coupled to absorb the pump light from the pump laser. The amount of pump light absorbed in the fiber amplifier is adjustable so as to control a gain tilt of the fiber amplifier.